The transducing heads are one of the key elements of the modern disk-type data recording device. Briefly, such disk memories typically comprise a thin disk spun at high speeds, whose surfaces carry a recording medium, typically powdered iron oxide. Data is recorded on and read from the disk surfaces one track at a time by a movable recording head, which is suspended from an arm and which follows a generally radial (with respect to the disk) path while accessing one or another track. A common means of increasing the amount of data which a single disk surface can store is to reduce the track width, so that more tracks become available. This requires that the width of the transducing surface of the magnetic head be decreased. To a certain point, conventional manufacturing techniques and head core designs suffice in achieving this. Now, however, tracks are packed at nearly 400 per centimeter of radius, which requires that the width of the recording gap in the transducing surface be 0.025 mm. or less. Experience shows that if the entire core is machined to 0.025 mm., it usually saturates, limiting the linear bit density in each track. Further, such cores are extremely fragile.
The solution is to make the transducing face of the head core relatively narrow as compared to the thickness of the body of the core. Our experimentation has further shown that simply beveling the core on one or both sides to form the narrowed face is not satisfactory because fringing occurs during use of such a core, causing track width to be greater than transducing face width and unduly dependent upon spacing of the face from the recording medium. The solution is to make the transducing core with a relatively narrow projection having substantially parallel sides which define the transducing face, and a relatively thicker portion integral with the projection which provides the magnetic circuit path.
Another factor which we have found important is the problem of achieving repeatability of transducing face width. If the width of the face varies with respect to its length, as in currently produced cores, it is difficult to accurately control the width of the transducing gap, and the result is that a relatively large number of cores will be rejected because they are too narrow or too wide. Our investigation has shown that this problem is caused by inability of current machining methods to grind away parts of individual ferrite grains which form these cores without exceeding the inter-grain binding forces. If the machining process causes these forces to be exceeded, then whole grains will be dislodged. Since the size of individual grains is a significant fraction of the width of the transducing face, dislodging of entire grains rather than partial abrasion of them produces significant deviations in face width. Furthermore, because of the fringing problem, the surfaces defining the width of the transducing face must extend substantially past the depth, or throat apex, of the flux gap. Removal of substantial numbers of ferrite grains in toto from these surfaces will weaken the projection which they define so that it can easily break off during the final portions of the machining operation or during later assembly operations. Furthermore, removal of these grains in the critical areas adjacent the gap may adversely affect the reluctance of the core.